1. Field of the Invention
The present invention relates to an exposure apparatus and a method for manufacturing a device.
2. Description of the Related Art
Conventional exposure apparatuses for manufacturing semiconductor integrated circuits use light of various wavelengths for exposure. The followings are used for exposure: an i-line with a wavelength of 365 nm, a KrF excimer laser beam with a wavelength of 248 nm, an ArF excimer laser beam with a wavelength of 193 nm, and X-rays. Light emitted from a light source passes through a projection optical system, including projection lenses, for projecting a pattern formed on an original onto a substrate. This forms the pattern on the substrate. The conventional exposure apparatuses need to have high throughput and high resolution because of the reduction in the width of pattern lines. Therefore, high-energy exposure sources are being demanded and light with a shorter wavelength is also being demanded for exposure.
Exposure light with a short wavelength is known to cause the photochemical reaction between oxygen and impurities present in the exposure apparatuses. The products of the photochemical reaction, that is, chemical contaminants adhere to optical elements, such as lenses and mirrors, included in optical systems disposed in the exposure apparatuses. This causes a problem that fog is caused in the exposure light. Examples of the impurities include basic gases derived from resists and organic silicon compounds, such as siloxanes, derived from adhesives used for the optical elements. The impurities cover gaseous substances and solid substances, for example, suspended particulate matter.
An exposure apparatus is usually placed in a chamber. The chamber includes a gas temperature control unit, an ultra-low penetration air (ULPA) filter, and a chemical filter. Gas supplied to the chamber is maintained at a constant temperature and has an extremely low impurity content.
There are many impurity sources in the exposure apparatus. For example, an adhesive is used to fix optical elements to supporting members disposed in a lens barrel. The adhesive is usually an elastic adhesive, which usually contains a siloxane. The siloxane can cause fog in these optical elements. Therefore, the concentration of impurities in the lens barrel needs to be low.
In order to solve the problem, Japanese Patent Laid-Open No. 11-145053 proposes a lens barrel 220 shown in FIG. 20. The lens barrel 220 includes lenses 230 and supporting members 231, having openings 232, for supporting the lenses 230. This configuration allows gas to flow through spaces partitioned by the lenses 230 and the supporting members 231. The lens barrel 220 is connected to a supply unit 206 and a recovery unit 207; hence, the atmosphere in the lens barrel 220 can be replaced with a purge gas.
Japanese Patent Laid-Open No. 2005-183624 proposes lens barrels 123 shown in FIG. 21. The lens barrels 123 include gas guides 125 for guiding a purge gas G to lenses 118 having a gap L1 therebetween. This configuration is effective in efficiently removing impurities from the lens barrels 123.
In the lens barrel 220 disclosed in Japanese Patent Laid-Open No. 11-145053, although the openings 232 are arranged around each lens 230 as shown in FIG. 20, the purge gas hardly flows on the lens 230. Therefore, if impurities causing fog are produced in the lens barrel 220, the concentration of the impurities present on the lens 230 is high because of diffusion and cannot be sufficiently reduced. In the lens barrels 123 disclosed in Japanese Patent Laid-Open No. 2005-183624, the purge gas G flows on the lenses 118 as shown in FIG. 21. An increase in the amount of the purge gas G flowing on each lens 118 causes an increase in the amount of chemical contaminants. Therefore, the lens barrels 123 disclosed in Japanese Patent Laid-Open No. 2005-183624 are ineffective in preventing the fog of the lens 118.
Lenses usually have an axisymmetric spherical surface or axisymmetric aspherical surface. Therefore, although the temperature distribution in the lenses or the distortion thereof may deteriorate optical properties of optical systems, the optical aberration caused by the temperature distribution or the distortion can be readily corrected when the temperature distribution or the distortion is axisymmetric. However, in the lens barrels 123 disclosed in Japanese Patent Laid-Open No. 2005-183624, if the purge gas G flows on each lens 118 in one direction, the temperature distribution in the lens 118 is not axisymmetric; hence, it is difficult to correct the optical aberration caused by the temperature distribution.